Did it matter Huck was introduced in The Adventures of Tom Sawyer? Not really. The characters were reintroduced throughout the novel, personality traits and habits written in where necessary. And Twain didn’t rehash the plot because it was insignificant to the movement of the second story. So done and done.
But some stories do need the background. For example, The Hunger Games sequel Catching Fire wouldn’t make sense without the referencing the plot of the first book. Rehashing the previous plot too much will lead to boredom for returning readers. But avoiding an explanation may lead to confusion for new readers. Tough call.